Conventionally, non-aqueous electrolyte secondary batteries that use a transition metal oxide as a positive electrode active material and a layered carbon compound as a negative electrode active material have been developed. As the transition metal oxide, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4) and the like are used. As the layered carbon compound, artificial graphite, natural graphite and the like are used.
In general, non-aqueous electrolyte secondary batteries have a tendency to exhibit good characteristics during the initial period of use, but the characteristics gradually degrade with repeated charging and discharging. Therefore, there is a demand for a highly reliable non-aqueous electrolyte secondary battery capable of maintaining good characteristics for a long period. Examples of the indices of the reliability of non-aqueous electrolyte secondary batteries include cycle life characteristics, high-temperature storage characteristics and load characteristics. Side reactions that occur at a contact surface between the positive electrode and the non-aqueous electrolyte (hereinafter, referred to as a “positive electrode interface”), and a contact surface between the negative electrode and the non-aqueous electrolyte (hereinafter, referred to as a “negative electrode interface”) affect these characteristics. The ion diffusivity in the non-aqueous electrolyte also affects these characteristics. Thus, the non-aqueous electrolyte affects the reliability of the non-aqueous electrolyte secondary battery.
A non-aqueous electrolyte contains a lithium salt and a non-aqueous solvent. As the non-aqueous solvent, for example, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), linear carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) are widely used.
Cyclic carbonates dissolve and dissociate a lithium salt into ions, thereby imparting the non-aqueous electrolyte with ion conductivity. More specifically, LiPF6, for example, is dissociated into a Li+ ion and a PF6− ion. However, cyclic carbonates are disadvantageous in that they have high viscosity and have low wettability to polyolefin separators. On the other hand, linear carbonates have low viscosity and have excellent wettability to separators. For this reason, a mixed solvent of a cyclic carbonate and a linear carbonate that overcomes the disadvantage of the cyclic carbonate is widely used as a non-aqueous solvent.
When a non-aqueous electrolyte secondary battery is stored at a high temperature, a mixed solvent of a cyclic carbonate and a linear carbonate occasionally generates a gas. The reason is that the linear carbonate is decomposed at the negative electrode interface in the initial period of high-temperature storage, which triggers a chain of decompositions through the non-aqueous electrolyte both at the positive electrode interface and the negative electrode interface.
As a technology for solving such a problem, for example, U.S. Patent Application Publication No. 2007/0054186 (hereinafter, referred to as “Patent Document 1”) discloses a fluorinated ether (2a) represented by the following Formula (2a):CF3CFHCF2CH(CH3)—O—CF2CFHCF3  (2a).
Patent Document 1 discloses that a lithium ion conductive coating is formed at a negative electrode interface when the fluorinated ether (2a) is added to a non-aqueous solvent mainly composed of a linear carbonate. This seems to suppress the decomposition of the linear carbonate at the negative electrode interface. However, in order to sustain this effect for a long period, it is necessary to set the content of the fluorinated ether (2a) to about 20 mol % of the total amount of the non-aqueous solvent. A secondary battery that uses a non-aqueous solvent mainly composed of a linear carbonate and containing 20 mol % of the fluorinated ether (2a) has the problem of having low load characteristics.
On the other hand, Japanese Laid-Open Patent Publication No. Hei 11-026015 (hereinafter, referred to as “Patent Document 2”) discloses a non-aqueous electrolyte containing a fluorinated ether (A) represented by the following Formula:HCF2CF2CH2—O—CF2CF2H  (A).
Patent Document 2 discloses a non-aqueous solvent mainly composed of a cyclic carbonate and in which the content of a linear carbonate has been reduced by adding the fluorinated ether (A). Patent Document 2 discloses that the battery load characteristics are improved by setting the content of the fluorinated ether (A) to 20 mol % or more of the total amount of the non-aqueous solvent.